1. Field of the Invention
The present invention relates to an optical pickup actuator and an optical recording and/or reproducing apparatus using the same, and more particularly, to an optical pickup actuator reducing an access time to a disc by inserting a damping member into a yoke to improve a damping effect, and an optical recording and/or reproducing apparatus using the same.
2. Description of the Related Art
Access time to discs is one of most important performance characteristics of high-speed optical recording and/or reproducing apparatuses. It has been proposed that a Q factor, that is, a damping factor of optical pickup actuators, be reduced in order to reduce the access time. In general, optical pickup actuators drive optical pickups for recording and/or reproducing information on, and/or from, optical information storage media such as optical discs. Further, the optical pickup actuators perform focusing servo, and tracking servo, of the optical pickups due to an electronic interaction between a magnet and tracking coils, and an electronic interaction between the magnet and focusing coils. In other words, focusing and tracking operations of an objective lens are performed by the optical pickup actuators.
Referring to FIG. 1, in a conventional optical pickup actuator, an objective lens 42 is mounted on a bobbin 40. The bobbin 40 is movably supported by suspension wires 44a and 44b, each of which has one end fixed to a holder 14. A magnetic driving unit is provided driving the bobbin 40 in focusing and tracking directions.
The magnetic driving unit includes focusing coils 54 wound around the bobbin 40, tracking coils 52a and 52b wound perpendicularly to the focusing coils 54, and magnets 30 and 30′, attached to yokes 20 and 20′, opposite to the focusing coils 54 and the tracking coils 52a and 52b. If power is applied to the focusing coils 54 or the tracking coils 52a and 52b, the bobbin 40 is driven in a focusing or tracking direction due to an interaction between the magnets 30 and 30′ and the focusing coils 54, or an interaction between the magnets 30 and 30′ and the tracking coils 52a and 52b, so that focusing and tracking operations of the objective lens 42 are performed.
Since the bobbin 40 is suspended in the holder 14, attached to base 12 with fasteners 16a and 16b, by the suspension wires 44a and 44b in a conventional optical pickup actuator, a vibration occurs during the focusing and tracking operations of the objective lens 42 so that a long time for stably moving the objective lens 42 to a target position is required. This will be described in detail with reference to FIG. 2.
In FIG. 2, a vertical axis s indicates a moving distance of the objective lens 42 to reach a target position, and a horizontal axis t indicates a time required for moving the objective lens 42 to the target position. Ga is a line indicating an ideal movement of the objective lens 42, where, the time for the objective lens 42 to reach a target position T is “0”. However, this is practically impossible because a speed (that is, a gradient of the line Ga) of the objective lens 42 would be instantaneously infinite value. Gb indicates a line describing an actual movement of the objective lens 42, where the time for the objective lens 42 to reach the target position T is “tb.” If a moving speed of the objective lens 42 increases in order to reduce the time “tb,” the time when the objective lens 42 reaches the target position T can be reduced from “tb” to “tc” or “td” as indicated in plots “Gc” or “Gd,” respectively. However, as the moving speed of the objective lens 42 increases, vibration of the objective lens 42 increases.
As described above, the time required for moving the objective lens 42 to the target position T is a factor in determining the access time of a recording apparatus. In order to reduce the access time, attempts to reduce a Q factor have been made. Conventionally, a damper bond is injected into a holder in an attempt to reduce the Q factor.
In FIG. 1, a damper bond 60 is injected into the holder 14, and thus, damping is achieved by the damper bond 60. A damping force is determined by a relative speed v between the damper bond 60 and the suspension wires 44a and 44b, and a damping factor c. The damping force f is expressed by f=cv. If the damper bond 60 is far away from fixed points of the suspension wires 44a and 44b, displacements of the suspension wires 44a and 44b are great. Thus, the relative speed v increases, and as a result, the damping force increases. If damping is achieved at a position near the fixed points of the suspension wires 44a and 44b, the relative speed between the suspension wires 44a and 44b and the damper bond 60 is less than a moving speed of the bobbin 40, and thus, the damping force is reduced.
When the damping force increases and the Q factor decreases, the access time to a disc drive can be reduced. However, in the conventional optical pickup actuator shown in FIG. 1, since the damper bond 60 is positioned near the fixed points of the suspension wires 44a and 44b, the damping force is small. Thus, it is difficult to reduce the Q factor to 10 dB, or less, due to generation of overdamping, and thus the reduction of the access time is limited.
A method of reducing the access time, for some actuators, was developed by injecting magnetic fluid, having magnetic flux density of a proper intensity, between coils and a magnet and reducing a Q factor to a level of 0 dB.
Referring to FIG. 3, a magnetic fluid 70 is injected between a holder 14 and a bobbin 40. However, enclosing means for preventing the magnetic fluid 70 from leaking from the injected space is required. If the magnetic fluid 70 leaks, problems, such as pollution, occur frequently during manufacturing and when using the optical pickup actuator.
The above problems are more serious for a high-speed actuator. Further, a high-speed actuator uses a multipolar magnet instead of a general magnet and fine pattern coils. There is no known method for injecting magnetic fluid into a high-speed actuator having a multipolar magnet and fine pattern coils. Thus, the magnetic fluid cannot be used in the high-speed actuator.
Further, three objective lenses are frequently used in an actuator for a HD-DVD. However, as a total weight of the objective lenses is increased, DC sensitivity and AC sensitivity decrease. For example, a weight of an objective lens is about 100 mg in an actuator for a HD-DVD, while a weight of an objective lens is about 14 mg in an actuator for a CD-RW, and a weight of an objective lens is about 25 mg in an actuator for a DVD. Since the DC sensitivity and the AC sensitivity are decreased due to an increase in a weight of an objective lens, a weight of a bobbin must be reduced to compensate for the increased weight of the objective lens.